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Split by test examples


Only RSpec >= 3.3.0 is supported. Let us know if you use a different test runner. As an alternative, consider:

  • spreading test examples into multiple files
  • tagging test examples (e.g., RSpec's --tag)

You can set KNAPSACK_PRO_RSPEC_SPLIT_BY_TEST_EXAMPLES to parallelize tests across CI nodes by example (it/specify). This is useful when you have slow test files but don't want to manually split test examples into smaller test files.

As an example, imagine you have two test files in your suite:

Test file path (2 files)Time execution

6 minutes and 30 seconds


2 minutes and 30 seconds

On your Knapsack Pro dashboard, you can see the yellow highlight because of the bottleneck: if you run those tests on 2 parallel CI nodes, the total execution time would be 6.5 minutes instead of the optimal 4.5 minutes (2.5 minutes + 6.5 minutes divided by 2 CI nodes).

By enabling KNAPSACK_PRO_RSPEC_SPLIT_BY_TEST_EXAMPLES, the bottleneck disappears because Knapsack Pro can distribute tests so that each CI node is balanced (e.g., 4.5 minutes + 4.5 minutes):

Test file path (5 files)Time execution

2 minutes


2 minutes


2 minutes


30 seconds


2 minutes and 30 seconds

Knapsack Pro knows the optimal time per parallel CI node is 4.5 minutes, so it looks for test files slower than 3.15 minutes (70% of 4.5 minutes) and splits them by test examples across parallel CI nodes. We use 70% as a threshold for two reasons:

  • Test execution time varies–especially for end-to-end tests–and we want to split all the potential bottlenecks.
  • RSpec consumes a lot of memory running individual test examples, so Knapsack Pro only splits potential bottlenecks and parallelizes the rest of your test suite by file.

In other words, files are split by test examples just enough to guarantee all the parallel CI nodes finish at a similar time to maximize performance and minimize memory consumption.

We recommend running at least 2 CI builds after you enable this feature or change the number of CI nodes to allow the Knapsack Pro API to learn about your test suite.


Does not support --tag

Due to the RSpec internals, --tag might be ignored when used together with KNAPSACK_PRO_RSPEC_SPLIT_BY_TEST_EXAMPLES. But you can use the KNAPSACK_PRO_TEST_FILE_* environment variables to filter the test files to run.



Why are some of my test files not executed?

Read the answer on the RSpec page.

How do I fix LoadError: cannot load such file -- MY_RUBY_GEM?

Probably, you load MY_RUBY_GEM in Rakefile so when knapsack_pro runs a rake task it fails. To fix the problem either:

  • Don't load MY_RUBY_GEM when RAILS_ENV=test
  • Add MY_RUBY_GEM in the :test group in the Gemfile

How do I fix Don't know how to build task 'knapsack_pro:rspec_test_example_detector'?

Try to remove the default prefix bundle exec used by knapsack_pro by setting KNAPSACK_PRO_RSPEC_TEST_EXAMPLE_DETECTOR_PREFIX="".

How do I fix Could not generate JSON report for RSpec. Rake task failed when running RACK_ENV=test RAILS_ENV=test bundle exec rake knapsack_pro:rspec_test_example_detector?

Most likely, RSpec is causing your CI machine to freeze due to a lack of resources: you could try to use fewer parallel nodes, or add more CPU/RAM. There is nothing we can do on our side, sorry!

Check the terminal output for the actionable error message:

E, [2021-03-30T17:33:12.199274 #103] ERROR -- : [knapsack_pro] ---------- START of actionable error message --------------------------------------------------
E, [2021-03-30T17:33:12.199329 #103] ERROR -- : [knapsack_pro] There was a problem while generating test examples for the slow test files using the RSpec dry-run flag. To reproduce the error triggered by the RSpec, please try to run below command (this way, you can find out what is causing the error):
E, [2021-03-30T17:33:12.199348 #103] ERROR -- : [knapsack_pro] bundle exec rspec --format json --dry-run --out .knapsack_pro/test_case_detectors/rspec/rspec_dry_run_json_report_node_0.json --default-path spec spec/models/user_spec.rb spec/features/articles_spec.rb
E, [2021-03-30T17:33:12.199368 #103] ERROR -- : [knapsack_pro] ---------- END of actionable error message --------------------------------------------------